1. Field of the Invention
The present invention relates to a method and apparatus for establishing and maintaining frame synchronization in a mobile satellite communication system, and more particularly, for establishing and maintaining frame synchronization on the basis of unique word (UW) detection to prevent erroneous operation due to shadowing phenomena occurring on the transmission path.
2. Description of the Related Art
In satellite communication systems, forward error correction (FEC) techniques with high coding gain or high-efficiency speech coding are used to reduce the required transmission power and raise the channel capacity. Usually, such satellite communication systems are operated at extremely low carrier-power to noise-power (C/N) ratios. Therefore, in a satellite communication system employing high-coding gain error correction or high-efficiency speech coding techniques, it is desirable to maintain stable frame synchronization at such low C/N ratios.
In order to access a satellite channel and thereby link the satellite channel to a station, it is necessary to synchronize the transmission and receiving timing of the frames in the signal. That is, the rate at which the frames of the communication signal are transmitted by the satellite or stations must be synchronized with the rate at which such frames are detected by the satellite or station.
Such frame synchronization can typically be accomplished by using a unique word (UW) in a preamble added to the initial portion of a burst signal, or unique words periodically provided in frames of the signal. The unique word or words used for frame synchronization can also be used to resolve phase ambiguity of the communication signal, or identify a satellite channel.
A number of different frame synchronization systems for such purposes have been proposed (see, for example, "Frame Synchronization", Japanese laid-open Patent Application No. Sho 61 (1986), and Japanese laid-open Patent No. Sho 62 (1987)-180634). Among these, the systems which establish frame synchronization by examining the demodulated bit stream on a bit-by-bit basis to detect the unique word are most common.
FIG. 9 illustrates a block diagram of a conventional frame synchronization system. As shown, the received modulated signals are supplied to a demodulator 1 and demodulated into baseband demodulated signals. The demodulated signals are supplied to a UW detector 6, where a unique word UW is detected from the demodulated signals. When a UW is detected in the UW detector, a frame synchronizing circuit 7 sets the timing for receiving the frames based on a predetermined, prestored frame format corresponding to the detected UW.
An aperture generator 8 controls the detection aperture of the UW detector 6 in accordance with the output signal of the frame synchronizing circuit 7. The aperture of the UW detector 6 is the time period that the UW detector remains "open" to detect the communication signal.
Thus, the UW detector 6, frame synchronizing circuit 7 and aperture generator 8 cooperate to establish frame synchronization by the procedure shown in the flow chart of FIG. 10. That is, in steps 101 and 102 of the frame synchronization process, the UW detector 6 remains "open" to receive the communication signal until it detects an initial unique word in the communication signal. UW detectors of this type are described, for example, in "TDMA Communication" by Heiichi Yamamoto et al., a 1989 publication by the Institute of Electronics, Information and Communication Engineers, and "Elements of Digital Satellite Communication Volume 1" by W. W. Wu which was published in 1984 by Computer Science Press.
In step 103, as explained above, when the UW detector 6 detects a unique word UW in the communication signal, the UW detector provides a signal to the frame synchronizing circuit 7, which sets the timing for receiving frames in the communication signal based on a predetermined frame format corresponding to the detected unique word UW. The frame synchronizing circuit 7 also controls the UW detector 6 via the aperture generator 8 to detect unique words in narrow apertures, that is, during short time periods.
The timing at which frames are received is established by setting the receive frame counter of the frame synchronizing circuit 7, which counts periods during which frames are received. The count of the receive frame counter is used as the time reference for various functions synchronization with received frames.
To determine the next time period during which the UW detector can detect the next unique word in the communication signal, the position in the communication signal of that next unique word is estimated based on the receive frame timing. The aperture generator 8 thus generates an aperture signal according to that position, and permit the UW detector to detect a unique word only during that time period designated by the aperture signal.
The aperture generator 8 continues to generate a series of aperture signals to enable the UW detector to detect subsequent unique words UW in the communication signal. Then, to assure that frame synchronization is occurring based on the unique words in the communication signal, in step 104, it is determined whether unique words are detected "I" consecutive times with this narrow aperture (brief time period). Unless unique words have been detected I consecutive times, the processing returns to step 101, and repeats. When it is determined that unique words have been detected "I" consecutive times, in step 105, it is determined that frame synchronization has been established.
After frame synchronization has been established, in step 106, the UW detector 6 is controlled by the frame synchronizing circuit 7 and aperture generator to continue detecting unique words during the same apertures (time periods) to maintain frame synchronization. However, if at any time during the UW detection process, the UW detector 6 does not detect unique words for "J" consecutive times (i.e. "J" consecutive aperture time periods), it is determined that frame synchronization has been lost. Accordingly, the processing returns to step 101 and is repeated as described above.
Note that the value "J" is used to assure that frame synchronization actually has been lost before the processing is repeated. That is, after frame synchronization has been established, if the UW detector 6 fails to detect a unique word during, for example, only one aperture period, frame synchronization is not considered to be lost. However, if the UW detector 6 fails to detect unique words during "J" consecutive aperture periods, synchronization is presumed to have been lost, and thus, the process of FIG. 10 is repeated to resynchronize the timing of the communication. The values of "I" and "J" usually depend on the type of satellite communication systems being operated. "I" is typically equal to any number from 2 to 4, and "J" is typically equal to any number from 2 to 6.
As described above, the conventional frame synchronization system detects UW's and establishes and maintains synchronization between the transmission and receipt of the frames in the communication signal according to the number of consecutive times that the UW detector detects or fails to detect unique words. However, the conditions of the transmission paths for mobile satellite communication stations used, for example, in motor vehicles, ships, aircraft and the like, differ from those of fixed satellite communication stations. For example, when a mobile earth station is used, the communication signals are subject to a shadowing phenomena caused by buildings, terrain, and other objects which may interfere with the signal transmission.
As described above, the conventional system merely estimates the position of the unique words in the communication signal after a first unique word is received. Hence, in essence, the frame synchronization is determined based on unique word detection irrespective of whether the actual communication signal continues robe received. Hence, the system may erroneously interpret portions of noise signals to be unique words when the actual communication signals have been interrupted. Thus, the system may erroneously set the timing of the receipt of the frames based on these erroneous unique words.
Such shadowing phenomenon and modulation timing errors may be anticipated when using mobile satellite communication systems. Hence, the aperture times of the UW detector 6 may be widened (i.e the time periods during which the UW detector detects unique words are increased) to compensate for the lag in frame timing after the shadowing phenomenon and/or modulation timing errors occur. However, as the aperture size (time period) is increased, the rate of false unique word detection also may increase. Thus, the frame timing may become erroneous.
Furthermore, once erroneous frame timing is set, even if the actual communication signals are again received by the demodulator 1 in the station, it is difficult to correct a signal processing system, such as a descrambler, that has already been erroneously synchronized. Hence, numerous resynchronization attempts may be necessary.
In addition, when the unique words are used also to resolve ambiguity concerning the phase of the communication signal, any erroneous unique words cause the phase of the communication signal to be erroneously determined. Hence, it may be impossible for the phase to be determined correctly.
Due to the problems that may result when using a unconventional frame synchronization system as described above, the quality of the communication signal may be substantially degraded. Accordingly, it is necessary to provide a system whose communication signal timing is not adversely affected by shadowing phenomena or the like.